Orthogonal frequency division multiplexing (OFDM) is a scheme for the transmission of digital data which utilises a large number of closely-spaced orthogonal subcarriers. In particular, digital information is effectively encoded into several parallel data streams, or channels, each of which is allocated to one subcarrier. The information is modulated onto the subcarriers using a conventional modulation scheme, such as quadrature amplitude modulation (QAM) or phase shift keying (PSK), at a symbol rate that is low, relative to the overall data transmission rate. The overall transmission capacity of OFDM systems is generally comparable to conventional single-carrier, or baseband, modulation schemes utilising the same bandwidth. A primary motivation for the use of OFDM in preference to single-carrier, or baseband, transmission is the ease with which channel equalisation may be performed. Equalisation is simplified because OFDM effectively employs many relatively slowly-modulated narrowband signals, rather than one rapidly-varying wideband signal. Furthermore, OFDM systems may be implemented very efficiently by utilising an inverse fast Fourier transform (FFT) at the transmitting end to generate the OFDM signal, and a forward FFT at the receiving end to recover the narrowband subcarrier channels.
OFDM has become particularly popular in wireless communications systems, because the duration of each symbol is relatively long compared to relevant time characteristics of radio channels, such as the differential delays caused by multipath propagation. As a result, OFDM signals suffer less from inter-symbol interference caused by multipath propagation than equivalent single-carrier signals. Indeed, by introducing a guard interval between consecutive OFDM symbols, inter-symbol interference may be eliminated altogether. So long as the duration of each symbol is long, compared with the guard interval, as is the case in typical wireless transmission systems, the resulting decrease in transmission efficiency is negligible.
In OFDM systems employing the FFT algorithm for modulation and demodulation, is advantageous to transmit a so-called “cyclic prefix” during the guard interval. Specifically, the guard interval precedes a corresponding OFDM symbol, and the cyclic prefix consists of a copy of the end portion of the OFDM symbol. This approach ensures that periodicity of the signal input to the FFT in the receiver is maintained, in the presence of inter-symbol interference having a total time spread that is less than the duration of the guard interval and cyclic prefix.
More recently, there has been increasing interest in utilising OFDM and related techniques within optical communications systems. One such approach, which also employs optical single sideband (OSSB) transmission via optical fibre, is disclosed in International Publication No. WO 2007/041799, which is incorporated herein in its entirety by reference. It has been recognised, in particular, that OFDM equalisation techniques may be employed to provide compensation within the electrical domain for the effects of chromatic dispersion occurring during transmission via single-mode optical fibres. Advantageously, electronic dispersion compensation methods enable optical links to be modified, reconfigured and/or upgraded without the need to replace or reconfigure outside plant, such as conventional optical dispersion compensators, eg dispersion-compensating fibre. The modifications necessary in order to compensate for changed dispersion characteristics occur only within the transmitter and/or receiver electronics, and in many cases may be performed without the need to replace hardware, and possibly even in a fully automated and adaptive manner without human intervention. This latter feature is particularly useful in dynamically-reconfigurable optical networks.
However, the deployment of OFDM within optical systems presents new challenges, because the relevant characteristics of typical transmitted optical signals are quite different from those of radio signals, in relation to which OFDM has so far most commonly been employed. In typical wireless systems, an OFDM signal may include on the order of hundreds to thousands of subcarriers, occupying a bandwidth on the order of hundreds of megahertz, with a subcarrier spacing of a few kilohertz, in order to achieve a net bit rate on the order of one to 10 megabits-per-second. In such systems, the symbol length is on the order of tens to hundreds of microseconds. In most applications, guard intervals having a duration of less than 25 percent of the symbol length may be used. Optical transmission systems, on the other hand, are commonly required to operate a bit rate of up to 10 Gbps, and beyond. An optical OFDM system employing hundreds of subcarriers over a bandwidth on the order of 10 gigahertz, has a subcarrier spacing in the range of tens to hundreds of megahertz. The typical symbol length is thus on the order of tens to hundreds of nanoseconds. Accordingly, if the inclusion of guard intervals is not to impact excessively on transmission efficiency, the duration of such intervals must be kept on the order of a few nanoseconds or less. However, since the extent of possible inter-symbol interference, and hence the required guard interval duration, is dependent upon characteristics of the optical channel, the required length of the guard interval may not be within the control of the system designer. Furthermore, a major source of inter-symbol interference in single-mode optical fibre transmission systems is chromatic dispersion, in accordance with which the differential delay experienced across the transmitted signal spectrum generally increases with increasing signal bandwidth. If, at the same time, the number of subcarriers is held constant, the bandwidth of each subcarrier will increase in proportion, and the symbol length will accordingly decrease. The required guard interval will therefore rapidly become comparable to the symbol length.
One solution would be to use a much larger number of subcarriers in optical OFDM systems. However this approach has a number of disadvantages. Firstly, the size of the FFT implementations used in the transmitter and receiver is necessarily equal to the number of subcarriers, and accordingly increasing the number of subcarriers increases the size of the FFTs required. This may be particularly disadvantageous in high-capacity transmission systems, wherein the FFTs are likely to be implemented in hardware, and accordingly the required silicon area, and wiring complexity, may be significantly increased. Additionally, some implementations of optical OFDM transmission utilise coherent receivers to convert the optical signal back into a corresponding electrical signal. Coherent detection is particularly susceptible to laser phase noise when narrowband subcarriers are used, and it is accordingly preferable to use a smaller number of wider-bandwidth subcarriers in such systems. As noted above, this is incompatible with the use of longer symbols in order to reduce the overhead due to required guard intervals.
Accordingly, it is an object of the present invention to provide methods and apparatus enabling improved efficiency of data transmission in optical OFDM systems, by reducing the duration of guard intervals required relative to the symbol length.